"getAverageGroundLevel()" does not return anything useful as average ground level.
It is 64 in the overworld; "dry" land is seen at 63 in shallow dips, and the overworld's ground level is well above 64 in most places.
What you can say is that the "base terrain height", for a biome with a root height of 0, is 64. There's a (roughly, approximately) -1 to +7 noise on top of that -- and yes, I know that it's really a 3d noise map. The *effect* in a biome with both root height and variation of 0 is just that, however.
BUT:
1. It's 32 in the nether, the nether sea is at 31, and ... there's nothing special about Y=32 terraingen over there.
2. It's 32 in Twilight Forest, and the sea level height in swamps is higher than the sea level height elsewhere ... so rainfall has to break one of those two.
3. It's ... 49? 50? in the end, and mystcraft island worlds. And the terrain actually changes at that point (it's where the ... "land sections", I guess, switch curvature). And Mystcraft actually has oceans there at AGL-1 so you get sugar cane, etc.
So: It approximates sea level, but it can be -1 or -2, depending; it generally specifies a "base ground level", but not always (cave worlds/nether); it determines your spawn height, and vanilla uses it for placing some structures (which can bury some if the ground goes hilly enough).
Not sure if all the other comments are directed at me, but the only things I use AGL for is underground determination and calculating fog density based on elevation.
I'm not really directing anything at anyone. I'm just frustrated at trying to deal with forge names that don't really reflect reality, and since this is the "suggest mods" thread, and since we can't actually make use of the MCP names in anything but forge, and they're basically borked, ...
(And, I'm frustrated by the limitations in the rain system I implemented).